[Q] Indentation rules for GRUB source code

I tried to find all relevant docs and read what I've found (e.g.
grub-dev.texi), so please excuse me if this questions has been already
answered somewhere.
I would like to change and submit code for GRUB and from what i see
right now indentation of the code doesn't seem to follow any rules. Most
of the time the visible indentation is 2 spaces which is either achieved
by using a TAB (which needs that the editor is configured to show TABs
as 2 spaces) or it's done by just using 2 spaces.
Regarding to what code I've seen until now, the indentation of quite a
lot of code is messed up and makes reading more difficult.
What are the preferred rules to follow when submitting changes or new
code regarding indentation?
BTW, it might be a good idea to propose the TAB setting configurations
for the most popular editors, e.g. I'm using vim right now.
Andreas